Sheldon hires an assistant named Alex, an attractive female grad student. Amy gets incredibly jealous of Sheldon having a female assistant, but it turns out that Alex is actually more interested in Leonard. Penny doesn't quite know what to do, since this does make her a tad jealous, so she "marks her territory" by sleeping with Leonard. Meanwhile, Wolowitz' return from space has been delayed a week and he starts to lose his mind because of it. He'll eventually make it back to earth in #604 and they'll deal with the ramifications of things that have changed in his absence.
